As a filler for providing electric conductivity to a resin or as an electric conductivity-providing material in an electrode of various kind of batteries, especially lithium-ion battery, acetylene black and carbon nanofibers (hereafter, referred to as CNFs), which are electrically conductive carbon materials, as well as a mixture thereof are used. In particular, using or adding CNFs leads to a preferable result that a high electric conductivity is achieved with a relatively low content of the electrically conductive carbon material, and thus, hopes are concentrated thereon. Here, a CNF generally has a fibrous form having an external diameter of 5 to 100 nm and an aspect ratio, which is a ratio of fiber length to external diameter, of 10 or more.
Conventionally, in manufacturing CNFs, an electrode discharging method, a catalytic vapor-phase growth method, a laser method and the like are employed, among which the catalytic vapor-phase growth method is recognized as most suitable for an industrial manufacturing method. According to the catalytic vapor-phase growth method, transition metal particles are used as a catalyst, which are brought into contact with a raw material gas of a carbon source such as acetylene and benzene at an elevated temperature of generally 900° C. or more, to grow CNFs from the catalyst particles. Particularly, a method to manufacture CNFs, using as a catalyst a transition metal component such as cobalt, from gas containing chiefly carbon monoxide as a raw material attracts attention as a method to produce CNFs of high purity and high quality at a relatively low temperature (Patent Documents 1 to 5).